Growing an Embryo from a Single Cell: a Hurdle in Animal Life
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Downloaded from http://cshperspectives.cshlp.org/ on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press Growing an Embryo from a Single Cell: A Hurdle in Animal Life Patrick H. O’Farrell Department of Biochemistry, University of California San Francisco, San Francisco, California 94158 Correspondence: [email protected] A requirement that an animal be able to feed to grow constrains how a cell can grow into an animal, and it forces an alternation between growth (increase in mass) and proliferation (increase in cell number). A growth-only phase that transforms a stem cell of ordinary proportions into a huge cell, the oocyte, requires dramatic adaptations to help a nucleus direct a 105-fold expansion of cytoplasmic volume. Proliferation without growth transforms the huge egg into an embryo while still accommodating an impotent nucleus overwhelmed by the voluminous cytoplasm. This growth program characterizes animals that deposit their eggs externally, but it is changed in mammals and in endoparasites. In these organisms, development in a nutritive environment releases the growth constraint, but growth of cells before gastrulation requires a new program to sustain pluripotency during this growth. he phrase “Nothing in biology makes sense scribe four almost universal phases of growth Texcept in the light of evolution,” originally and proliferation in animal biology. I will be introduced in an essay supporting the theory focusing on the two earliest phases: the growth of evolution (Dobzhansky 1973), has been re- that produces a huge egg and the transformation purposed to chide biologists, who all too often of this single large cell into an embryo. The sub- ignore the origin and purpose of the biologi- sequent phases of growth were described in pre- cal processes and mechanisms they study. To vious reviews (O’Farrell 2004, 2011; O’Farrell look for how these processes “make sense,” one et al. 2004). But first, I will provide some context should look at theway theyserve the life histories for the comparisons that are made, and a brief of the organisms in which they are used. Such an synopsis of the four growth phases that convert examination can suggest how a process might an ordinary-sized stem cell into a big metazoan have been molded by evolution to achieve the adult. benefit provided. I have been an advocate for such considerations, because I believe them to MAMMALS CAME LATE be the best guide to interesting and important questions for investigation. Here, I will synthe- It is perhaps important to point out to the read- size observations made by examination of the er that I will first focus on animals that deposit natural histories of diverse organisms to de- their eggs externally, as this oviparous lifestyle Editors: Rebecca Heald, Iswar K. Hariharan, and David B. Wake Additional Perspectives on Size Control in Biology: From Organelles to Organisms available at www.cshperspectives.org Copyright # 2015 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a019042 Cite this article as Cold Spring Harb Perspect Biol 2015;7:a019042 1 Downloaded from http://cshperspectives.cshlp.org/ on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press P.H. O’Farrell is the most general and ancestral mode of ani- ALIGNING PROGRAMS OF DEVELOPMENT mal development. Mammals, of course, are in- AT A CONSERVED STAGE teresting to us, but if we wish to understand our connection to the rest of biology, we should The strategy used when comparing distantly re- first recognize the context within which mam- lated protein sequences suggests a general ap- mals appeared. Mammals are a small clade of proach when looking for distant homologies. 4000 species out of more than two million Rather than just aligning sequences at the amino animal species, and they are derived chordates terminus, one first identifies the most conserved that arose relatively late in animal evolution. domains and uses these for alignment. Similarly, Furthermore, mammalian evolution invested in comparing the programs of growth and pro- in a special program, which, by housing embry- liferation of diverse species, I have chosen to os in a nutritive environment, introduced an align the life histories of different organisms at unusual fashion of dealing with the growth con- a particularly conserved point (O’Farrell 2004; straints faced by all embryos. Understanding the O’Farrell et al. 2004). specializations of mammalian development in Theearlydevelopmental biologists,vonBaer this context can be illuminating (O’Farrell et al. and Haeckel, were fascinated by the similarities 2004). I will end with an effort to point out of vertebrate embryos of very diverse organisms these mammalian specializations and deep con- at a stage following establishment of the body nections to programs showed by their evolu- axis and formation of the neural tube. Recogni- tionary predecessors. But we will begin with a tion of this conserved stage and description of few guiding generalizations that give perspective the morphogenetic events that brought increas- on the coordination of growth with animal de- ingly dramatic distinctions to the embryos of velopment. different species were taken by some as evidence for a claim that ontogeny recapitulates phyloge- ny—or that the developmental sequence passes through all of the evolutionary steps as if “more AN ORGANISM MUST DEVELOP FEEDING advanced” organisms achieve their distinctions STRUCTURES BEFORE IT CAN INCREASE by late additions to the developmental sequence IN MASS (e.g., Graham andRichardson2012). Theattrac- Although there are some exceptionally large tive phrase, “ontogeny recapitulates phylogeny,” single-celled organisms, large body plans are appears to be somewhat of an overstatement and substantially the domain of multicellular organ- the idea is often described as discredited. None- isms. However, the production of a large body theless, the existence of a relatively similar em- from a single-cell zygote must deal with a fun- bryonic morphology is strongly supported. damental problem. Any animal whose nutrition A common embryonic morphology is also depends on a complex body plan with its spe- found among embryos within different inverte- cialized feeding structures must be able to de- brate phyla. This is particularly apparent in ar- velop these specialized structures before it can thropods. Arthropod embryos of very diverse feed and grow independently. In these cases, a species show remarkably similar morphologies complex body plan needs to be produced dur- on the initial establishment of the body plan. ing development before there is significant These similar looking embryos also use similar growth of the organism (O’Farrell 2004). molecules to guide the formation of embryonic We will view life histories of organisms in patterns (Patel et al. 1989; Prud’homme et al. the light of this problem, which appears to have 2003). This point in development has been acted as a constraint throughout the evolution called the phylotypic stage to reflect the fact of the animals. Despite great diversity, this van- that diverse organisms within a phylum show a tage point reveals relationships that lead to the “typical morphology” at this stage. global concept of four phases of growth and It is notable that the “phylotypic” stages of proliferation in the life plans of animal species. different phyla occur at a similar point during 2 Cite this article as Cold Spring Harb Perspect Biol 2015;7:a019042 Downloaded from http://cshperspectives.cshlp.org/ on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press Growing an Embryo from a Single Cell embryogenesis, just after establishment of the same size, or relatively close (O’Farrell 2004). basic body plan, and embryos of even different Taking a little license with the exact stage that phyla have a very similar organization at this is defined as the phylotypic stage, the anterior/ stage. These distant similarities suggested by posterior axis of the phylotypic embryo is morphologies have been supported by molecu- roughly a millimeter. A few examples are a little lar analyses (Domazet-Losˇo and Tautz2010; Ka- smaller (e.g., Caenorhabditis elegans) or bigger linka et al. 2010). Key mechanisms patterning (e.g., grasshopper), and some embryos continue the phylotypicembryoareconserved across phy- to extend their length for some time after the la. For example, a gradient in a signaling mole- initial elaboration of the body axis. Nonetheless, cule of the transforming growth factor (TGF)-b sizes are rather similar. This similarity in size type guides dorsoventral polarity of the embryo, applies to the embryos of fruit flies, frogs, and and local expression of different homeotic-type blue whales, organisms whose adult masses, transcription factors subdivide the anteroposte- 1 mg, 100 g, and 100 metric tons, range over rior axis of the body plan (Shen 2007). These 1011-fold. This near-constancy in the size of findings suggest mechanistic parallels and argue the embryo at the phylotypic stage allows us to for evolution by descent. That is, even distantly divide our consideration of growth in animals related animals evolved from a common prede- into two. How do embryos of 10 mg(wet cessor with a similarembryonic stage, whichwas weight of cytoplasm) at the phylotypic stage patterned by mechanisms that continue to be grow to the adult sizes? How does oogenesis used in extant animals. Consequently, the phy- and early embryogenesis transform a single or- lotypic stage can be taken as an especially con- dinarily sized stem cell into a phylotypic em- served stage of development at which the embry- bryo, a miniorganism with a roughed-out body os of different species can be compared. plan? The original view exemplified by the “ontog- In this presentation, I consider growth as eny recapitulates phylogeny” phrase presumes an exponential process. Thus, the seemingly thatembryogenesisbeginswithacommonances- massive growth of a 7-ton blue whale calf into tral morphology that adds species-specific spe- a 150-ton adult is viewed as 20-fold and, cializations as embryogenesis progresses.